The present invention relates to rotary fluid pressure devices, and more particularly, to shaft seal assemblies for effecting a fluid seal between the rotary shaft and the housing of such devices.
Although the present invention may be used in many types of rotary fluid pressure devices, it is especially advantageous for use in low-speed, high-torque (LSHT) motors and will be described in connection therewith. In a typical LSHT hydraulic motor, high-pressure fluid is received by the motor, at an inlet port, and the fluid-pressure energy is translated into rotary motion of an output shaft by means of a displacement mechanism, such as a gerotor gear set. Such motors typically include a housing, surrounding the output shaft, and it is necessary to provide a fluid seal between the rotating shaft and the stationary housing to prevent pressurized fluid within the motor case from leaking out of the motor, between the shaft and the housing.
Various shaft seal assemblies have been used in the prior art, including such well known seal configurations as O-rings, quad rings, and various lip seal arrangements. Many of the well known shaft seal arrangements were generally satisfactory as long as the case drain fluid pressure, to which the shaft seal is subjected, is kept fairly low, for example, 50 PSI to 200 PSI. However, as LSHT motors have been improved, in terms of durability and torque-transmitting capacity, the fluid pressures communicated to such motors have gradually been increased, in many cases resulting in substantially greater case drain pressures.
In addition, in certain applications it is desirable to connect several LSHT motors in series, i.e., the outlet port of the first motor is connected to the inlet port of the second motor. Because case drain pressure is typically somewhere between motor inlet pressure and motor outlet pressure, the case drain pressure of the first motor in series may be somewhere in the range of 2,000 PSI, and it is necessary for the shaft seal assembly to be able to withstand such pressures over a period of many hours of operation.
Finally, in certain LSHT motor designs, the shaft seal assembly may be subjected directly to motor inlet pressure, at least in one direction of operation, i.e., in either clockwise or counter-clockwise rotation of the output shaft. In such motor designs, it is necessary for the shaft seal assembly to be able to withstand full motor inlet pressure which is frequently in the range of 3,000 PSI.
It is quite common for LSHT hydraulic motors to have their output shafts subjected to substantial side loading, resulting in radial movement or deflection of the shaft as it rotates. Such deflection is typically in the range of about 0.010 to about 0.030 inches. Such deflection of the output shaft makes it much more difficult to provide a shaft seal which is able to withstand relatively high pressure, and at the same time, continue to seal effectively over a relatively long period of operation. As the output shaft deflects, the result is increased loading of the seal member, at the point of deflection, and such increased loading results in more heat generation, and eventually leads to the seal member becoming brittle and losing its ability to maintain an effective oil seal. The seal may also be unable to follow the shaft deflection and lose contact with the shaft.